1. Field of the Invention
This invention relates to a process for producing terephthalic acid, which comprises oxidizing in the liquid phase in acetic acid a para-substituted aromatic compound with a gas containing molecular oxygen in the presence of oxidizing catalyst to obtain a reaction mixture in the form of a suspension containing terephthalic acid, and recovering the terephthalic acid crystals from said suspension, and more particularly relates to a process for said recovering.
2. Description of the Prior Art
In a typical known process for producing terephthalic acid, a para-dialkylbenzene, for example, is oxidized in acetic acid, used as solvent, with molecular oxygen in the presence of a catalyst. The para-dialkylbenzene generally used is para-xylene and the catalyst generally used is a cobalt compound and a manganese compound. An oxidation promoter such as a bromine compound, methyl ethyl ketone, paraldehyde or acetaldehyde is occasionally used. When para-xylene is oxidized in the liquid phase in acetic acid, used as solvent, and in the presence of a catalyst, the product terephthalic acid, being very difficultly soluble in acetic acid, crystallizes out of the solvent acetic acid, forming a suspension. The suspending medium, that is, the acetic acid contains a small amount of terephthalic acid dissolved therein, catalyst, unreacted para-xylene, intermediate oxidation products such as para-tolualdehyde, para-toluic acid, 4-carboxybenzaldehyde, and the like, an oxidation retarder such as para-cresol, and other organic impurities which cause discoloration. The main technical problems in recovering terephthalic acid from such an oxidation reaction mixture, that is, the suspension of terephthalic acid, arise from the following facts:
(1) Coprecipitation of dissolved impurities in the course of crystallization of dissolved terephthalic acid from the oxidation reaction mixture by cooling. PA1 (2) The terephthalic acid crystals in the oxidation reaction mixture, i.e. a hot suspension of terephthalic acid, have a broad particle size distribution and the smaller the particle size is, the larger is the impurity content of crystals. Minute crystals which precipitate on cooling the suspension have also a larger impurity content. PA1 (3) Coarse crystals in the terephthalic acid suspension also contain impurities.
Several methods had heretofore been proposed for recovering terephthalic acid from the suspension obtained as the oxidation reaction mixture, including those disclosed in Japanese Patent Publication No. 8,818/64 (A process for recovering crystals by gradually cooling the suspension taking long residence time by means of using several crystallizing tanks), Japanese Patent Publication No. 34,023/71 (A process comprising precipitating terephthalic acid by mixing with low temperature carboxylic acid solvent), Japanese Patent Application Laid-open ("Kokai") No. 135,939/74 (A process comprising separating terephthalic acid and mother liquor from reaction mixture at a temperature close to the reaction temperature) and Japanese Patent Application Laid-open ("Kokai") No. 91,835/77 (A process comprising additionally oxidizing the terephthalic acid suspension by molecular oxygen). Although each of the proposed methods solved the above problems to some extent, none was successful in solving all of the problems prior to the accomplishement of this invention.
The product terephthalic acid, which was produced according to the conventional method and subjected to solid-liquid separation and drying, has not satisfactory quality and requires further purifying treatment such as washing in order to obtain terephthalic acid pure enough for direct polymerization, and thus, these methods are complicated in steps and apparatus.
On the other hand, Japanese Patent Application Laid-open ("Kokai") No. 9,736/78 discloses a method in which the reaction mixture mother liquor is replaced with acetic acid at high temperatures by using a multi-stage cyclone unit and other means to separate the reaction mixture into a suspension of larger crystals in fresh acetic acid and mother liquor containing smaller crystals, the latter of which being recycled back to the reaction system. This is a common place but good method for solving the aforementioned problems of (1) and (2). However, it requires a complicated unit consisting of at least three stages of equipments (for example, cyclones) connected in series and is complicated also in operation. Moreover, it requires a large quantity of cold acetic acid for dilution so that the significant amount of impurities contained in the resulting suspension of larger crystals in fresh acetic acid may remain in the mother liquor without precipitating on being rapidly cooled and that the suspension may be cooled by the addition of fresh acetic acid. As is apparent from the foregoing description, none of the conventional methods is satisfactory and the fact is that in order to obtain highly pure terephthalic acid suitable for use in direct polymerization for production of so-called fiber-grade polyester, it is unavoidable to subject the crystallized terephthalic acid to an additional treatment using complicated equipments and complicated procedures.